Surface & Coatings Technology 228 (2013) S588–S592

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Improving the pilling property of knitted wool fabric with atmospheric pressureplasma treatment

C.W. Kan ⁎, C.W.M. Yuen, O.N. HungInstitute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

⁎ Corresponding author. Tel.: +852 2766 6531; fax:E-mail address: tccwk@inet.polyu.edu.hk (C.W. Kan

0257-8972/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2011.10.062

a b s t r a c t

a r t i c l e i n f o

Available online 3 December 2011

Keywords:Knitted wool fabricPlasmaPillingSurface

Knitted wool fabrics are very easy to pill during wear and use. In this study, knitted wool fabrics were treatedwith atmospheric pressure plasma (APP), a completely dry treatment, using plasma jet with helium andoxygen as carrier and reactive gas, respectively, under various processing parameters: (i) jet-to-substratedistance; (ii) oxygen flow rate; and (iii) treatment time. This paper reports the study of how these APPprocessing parameters influence pilling property of knitted wool fabrics. After APP treatment, pilling propertyof knitted wool fabrics was evaluated by standard testing method for pilling (EN ISO 12945-1; ICI pilling boxmethod). Also, a conventional wet anti-pilling treatment (with an anti-pilling agent) was carried out on knittedwool fabrics in order to compare anti-pilling effect of APP (dry treatment) and anti-pilling agent (wet treatment).Experimental results revealed that the APP treatment can greatly improve anti-pilling performancewithoutadverse effect like yellowing. Scanning electron microscopy revealed that surface etching effect induced byAPP treatment could be one of the reasons of improved anti-pilling property of knitted wool fabrics. Variousprocessing parameters provide different degrees of etching on wool fibre surface. Meanwhile, no fabricshrinkage was noted in the APP treated knitted wool fabric but about 5% shrinkage in area was noted in fabrictreated with anti-pilling agent.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Textile products made of wool have long established reputationfor comfort and quality due to the basic good mechanical propertiesof wool fibre. However, there are some technical problems whichaffect quality and performance of finished products such as feltingshrinkage, handle, lustre, pilling and dyeability. These problems maybe mainly due to the presence of wool scales on the fibre surface.The scales are fairly hard and there are sharp edges which are themain causes of fibre's directional movement and shrinkage duringfelting. The scales also serve as a barrier for diffusion processeswhich adversely affect the sorption behaviour [1].

In the past, many methods (mostly chemical) have been developedto counteract problems caused by the presence of scales. Owing to envi-ronmental consciousness, physical treatments such as plasmatreatment have been introduced recently as they are able to achieve asimilar descaling effect. In this study, knitted wool fabric, which pillseasily, was treated with atmospheric pressure plasma (APP) undervarying treatment conditions in terms of jet-to-substrate distance,

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oxygen flow rate and treatment time to study the influence of theseprocessing parameters on pilling of knitted wool fabrics [1,2]. Also, achemical anti-pilling treatment was carried out in order to compareeffects of APP and the conventional chemical anti-pilling treatment.Yellowness of wool fabrics was measured so as to evaluate the effecton aesthetic properties of the wool fabric.

2. Experimental

2.1. Wool fabric

Knitted wool fabric samples of size 20 cm×20 cm were used.Samples were cleaned with Soxhlet extraction with dichloromethanefor 4 h to remove any dirt and oily impurities. After that, the fabricswere dried at room temperature and conditioned for at least 24 hunder standard atmospheric condition (20±2 °C and 65±2% relativehumidity) prior to use.

2.2. Atmospheric pressure plasma (APP) treatment

APP treatment was imparted by an atmospheric pressure plasma jet(APPJ) mounted vertically above the substrate. The helium flow rateand discharge power were kept at 10 L/min and 80W respectivelyduring the treatment. The parameters used included oxygen flow rate

S589C.W. Kan et al. / Surface & Coatings Technology 228 (2013) S588–S592

(0.1 L/min, 0.3 L/min and 0.5 L/min), jet-to-substrate distance (1 mm,3 mmand 5 mm) and treatment time (1 s, 3 s and 5 s). The experimen-tal set-up is shown in Fig. 1. After APP treatment, wool fabrics wereconditioned for at least 24 h under standard atmospheric conditionprior to evaluation.

2.3. Anti-pilling treatment

The chemical used for anti-pilling treatment was Ultratex EMJ(Hunstman, US). Various concentrations of anti-pilling treatmentwere used: 1%, 2% and 3% on weight of fabric (o.w.f.). The anti-pillingagent was applied to wool fabrics by exhaustion method with aliquor-to-goods ratio of 20:1 and pH value of the treatment bath was4. The anti-pilling treatment was then conducted at 40 °C for 20 minwith very mild shaking to enhance the uptake of the anti-pilling agentbut with minimised shrinkage effect. After treatment, the liquor wasremoved from wool fabrics by hydro-extraction and dried completelyat 90 °C in an oven. The treated wool fabrics were conditioned for atleast 24 h before evaluation.

2.4. Scanning electron microscopy (SEM)

Surface morphology of the wool samples was examined by SEM —

Leica Stereoscan 440 (Leica Cambridge, England).

2.5. Measurement of anti-pilling performance

Anti-pilling performance of treated wool fabrics was evaluated bystandard testing method for pilling, EN ISO 12945-1 (ICI pilling boxmethod). After testing, fabric samples were visually assessed bycomparing themwith a set of photographic standards (subjective rating)on the following scale: 5 — no pilling; 4 — slight pilling; 3 — moderatepilling; 2 — severe pilling; and 1 — very severe pilling. Half grade wasacceptable for rating.

2.6. Measurement of yellowness

Yellowness index of wool fabric was measured with spectropho-tometer (Color-eye 7000) manufactured by Macbeth, in accordancewith test method ASTM E313. The illuminant used was D65 andobservation angle was 2°. Measurement was conducted on both faceand back of the fabric specimen.

2.7. Shrinkage measurement

A square of size 15 cm×15 m was marked initially in knitted woolfabrics and after different treatments, the sides of the square was

Fig. 1. Schematic diagram of APP treatment.

measured again. The percentage of area shrinkage was calculated byEq. (1).

Percentage of area shrinkage %ð Þ ¼ Ao−Afð Þ=Ao½ � � 100% 1

where

Ao initial area of the square (15 cm×15 cm=225 cm2)Af final area of the square after treatment

3. Results and discussion

3.1. Anti-pilling performance

The rating of untreated wool fabric was grade 3.5 while APP treatedfabrics and chemically treated wool fabrics were all in grade 5 whichmeans there was no pilling on treated wool fabrics. Since there wasno significant difference between wool fabrics with the two differenttreatments (APP and chemical), SEM was used for examiningdifferences in surfaces of the two samples [3] because the surfacestructure is believed to be a crucial factor in affecting the pilling propertyof wool fabric [1].

3.2. SEM analysis

Fig. 2 is the SEM image of untreated wool fabric which shows thatthe scales were intact, shape and round, implying the hydrophobicproperty. Presence of the epicuticle layer makes it difficult for thefibre surface to get wet [1]. Also, the non-uniform edges impedeliquid diffusion into the fibre. SEM images in Fig. 3(a) to (c) showfibre surfaces treated with different concentrations of anti-pillingagent. It is observed that there is a layer of agent coated on the surfaceof the fibres. The thickness of coating increased when concentrationof anti-pilling agent increased. Fig. 4 shows SEM images of wool fabricsurface treated under different treatment conditions. It is obvious thatthe APP treatment induced etching effect on the wool fibre surface,resulting in various degrees of surface roughening effect. This rough-ening effect on fibre surface induced by APP treatment can be used forexplaining the anti-pilling results. Pilling is a fabric-surface faultcharacterised by little ‘pills’ of entangled fibres clinging to the fabricsurface and giving the garment an unpleasant appearance. The pillsare easily formed during wear or washing because of entanglementof loose fibres which protrude from the fabric surface. After rubbing,these loose fibres develop into small spherical bundles anchored tothe fabric by a few unbroken fibres. Since the formation of pills isdue to migration of fibres from the constituent yarns in the fabric,in theory, reducing the migration tendency of fibres by APP treatmentcan help improving pilling resistance. In this study, significant

Fig. 2. SEM image of untreated wool.

Fig. 3. SEM images of anti-pilling agent treated wool: (a) 3%; (b) 2%; and (c) 1%.

S590 C.W. Kan et al. / Surface & Coatings Technology 228 (2013) S588–S592

changes in pilling resistance were found in APP treated fabrics, i.e. thepilling grade was changed from 3.5 to 5. The etching action on fibresurface increased the inter-fibre friction and, therefore, movementof fibres in yarns and the ability of the fibres to migrate out of theyarn surface to produce pilling balls during wear or washing werereduced. As a result, a reduction in the degree of pilling could beseen after APP treatment.

3.2.1. Effect of oxygen flow rate on wool fibre surface morphologySome micro pores appeared on the surface of wool fibres after APP

treatment and the number of micro pores increased with oxygen flowrate in the order 0.3 L/min>0.5 L/min>0.1 L/min. The oxygen flowrate can be considered as the concentration of oxygen used for APPtreatment. It is expected that 0.5 L/min would provide the mostsignificant plasma etching effect and produces more micro poresbut the experimental results revealed that 0.3 L/min imparts themost significant plasma etching effect among the different flowrates used. This phenomenon can be attributed to increase in oxygenflow rate increasing the supply of active plasma species for reactionon the wool fabric. However, if a large amount of oxygen is supplied

continuously, the active plasma species might react with the oxygeninstead of the wool fabric surface, resulting in the amount of activeplasma species getting reduced and lowering of the surface reaction[2]. As a result, 0.3 L/min was found to impart the most significantetching effect to wool fabric surface in APP treatment.

3.2.2. Effect of jet-to-substrate distance on wool fibre surface morphologyDensity of micro pores was the highest when the distance

between APP jet and fabric surface was 3 mm. The sequence ofdistance fromAPP jet to substrate from the highest to the lowest densityof micropores is 3 mm>5mm>1mm. When the distance was smallerthan 1 mm or larger than 5 mm, the effect of APP in terms of density ofmicro pores was not significant. When the distance between the APP jetnozzle and fabric surface was too small, the effect of APP treatment wasgreatly reduced as the gases were bounced off the fabric surface andflew out. When the distance reached 5 mm, the distance between jetnozzle and substrate surface is too large and the active plasma speciesrequire longer time to reach the fabric surface [2,4]. As a result, the effectof APP treatment was greatly reduced because the velocity and activityof active plasma species were greatly decreased by the travelling time[2,4].

3.2.3. Effect of treatment time on wool fibre surface morphologyNumber of micro pores found on fibre surface increased when

treatment time was increased. The order of the number of microporeswas 3 s>5 s>1 s. As the treatment time increased, concentration of ac-tive plasma species from the APP jet accumulating on the top increased.Once the concentration of active plasma species increased to a criticallevel, the reaction between active plasma species and wool fabricsurface gets saturated [5]. Excess active plasma species are thenneutralized by the surrounding air species because the plasma treatmentwas undergone in atmospheric pressure environment. Therefore, theresulting chemical and physical reactions induced by plasma treatmentare reduced and thus the etching effect is also reduced. Therefore, 3 sgives the most severe etching effect on wool fibre surface, according tothe results of this study.

3.3. Yellowness

The yellowness of the wool fabric is measured by a yellownessindex and the values are shown in Table 1. Index value of 100 impliesthe most yellowish hue in the fabric. In this study, the untreated andchemically treated wool fabrics have the same yellowness indexeswhich were −2.61 and −2.98 for the facing side and back siderespectively. The negative value of yellowness index indicates thatsurfaces of untreated and chemically treated (for anti-pilling) woolfabrics do not have any significant yellowing appearance. Also thechemical treatment did not impart yellowing effect in the woolfabrics. As shown in Table 1, yellowness index was increased afterAPP treatment on the face side but there was only a slightly increaseon the back side. Since the APP treatment is a downstream process(Fig. 1), the active plasma species bombard on the fabric surfaceand cause oxidation leading to surface yellowing [6]. It is expectedthat no yellowing effect on the back side of the wool fabrics becauseonly the face side of the wool fabric was treated with APP. Moreover,the substrate used in this study was a knitted fabric which has anumber of openings in the fabric structure. It is believed that fornonporous substrates, the APP treatment may only modify the surfacewhile in case of porous substrates, active plasma species maypenetrate through the openings and the back side of the fabric mayalso be affected [7,8].

3.3.1. Effect of oxygen flow rate on yellowness indexThe reactive concentration of oxygen affects the degree of plasma

reaction on fabric surface because oxygen plasma is chemically oxidativeand induces surface modification. In this study, 0.3 L/min induced the

1mm 3mm 5mm

1s

0.1L/min

0.3L/min

0.5L/min

3s

0. 1L/min

0.3L/min

0.5L/min

5s

0.1L/min

0.3L/min

0.5L/min

Fig. 4. SEM images of wool fibre surface under different treatment conditions.

Table 1Yellowness of APP treated wool fabrics.

Jet-to-substratedistance(mm)

1 s 3 s 5 s

0.1 L/min 0.3 L/min 0.5 L/min 0.1 L/min 0.3 L/min 0.5 L/min 0.1 L/min 0.3 L/min 0.5 L/min

f b f b f b f b f b f b f b f b f b

1 −1.08 −2.79 0.16 −0.50 −0.08 −1.11 −0.67 −0.56 0.66 −0.04 0.42 −0.09 −0.70 −2.10 0.57 −0.49 0.06 −0.813 −0.07 −0.02 3.47 0.84 0.66 0.01 3.03 1.70 8.70 5.20 6.97 2.43 2.78 0.87 5.18 3.27 4.20 1.315 −0.63 −1.03 1.15 −0.27 0.35 −0.35 −0.12 −0.11 3.53 1.32 1.63 0.53 −0.30 −0.46 2.30 0.63 0.47 0.48

f — face side; b — back side.

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highest yellowness on wool fabric. Generally speaking, a higher oxygenflow rate should cause amore severe surfacemodification on a substrate[9,10]. It is obvious that the lowest yellowness was achieved with 0.1 L/min. The experimental result, however, was to the contrary when0.5 L/min was used. This phenomenon can be explained by the plasmachemistry. O active species are not directly formed by the electricaldischarge in the APPJ. O active species are generated mainly by Penningreaction with helium metastables [11,12]. A higher oxygen flow ratemeans a higher oxygen concentration, which consumes more heliummetastables, and the reduced concentration of helium metastableshinders the sequential O active species production. Experimental resultssupport the above explanation. When the oxygen flow rate increasesduring the APP treatment, supply of active plasma species for reactionon thewool fabric is also increased. However, if a large amount of oxygenis supplied continuously, active plasma species might react with theoxygen instead of the wool fabric surface, thereby reducing the amountof active plasma species and lowering the surface reaction. Thus, 0.3 L/min was found to cause the most significant yellowing effect on woolfabric after APP treatment.

3.3.2. Effect of jet-to-substrate distance on yellowness indexThe interaction between active plasma species and fabric surface

is sensitive to the jet-to-substrate distance. The impact of jet-to-substrate distance on yellowing of wool fabric surface was greatly af-fected by density of the plasma gas. APPJ is a downstream treatment.The jet-to-substrate distance is the perpendicular distance betweenthe plasma jet and the substrate located below it as shown in Fig. 1.In general, an appropriate proximity between the plasma sourceand the substrate determines the efficacy of active plasma species inetching of a surface [2]. In APP treatment, active plasma speciesexperience a severe collision with air molecules when travellingtowards the substrate surface. The velocity and energy content ofthe active plasma species decrease with respect to time and distancetransported. As a result, 3 mm jet-to-substrate distance in this studygenerated the greatest yellowing effect on the wool fabric.

3.3.3. Effect of treatment time on yellowness indexTreatment time is also an essential processing parameter that

determines yellowing effect on wool fabric caused by APP. Accumulationof sufficient amount of active plasma species on the substrate for thereaction takes time. 1 second treatment time is not sufficient to inducesurfacemodification. Concentration of active plasma species from plasmajet accumulating on the surface increased in case of treatment for 5 s.Once the concentration of active plasma species increased to a criticallevel, the reaction between active plasma species and wool fabric surfacegets saturated [13]. The excess active plasma species are then neutralizedby the surrounding air species. Therefore, the resulting chemical andphysical reactions induced by plasma treatment are reduced and thusthe yellowing effect is also reduced. As a result, among all three treatmenttimes, 3 s provides the best plasma effect but causes the greatestyellowing effect. When the distance between the APP jet nozzle and thefabric surface was too small, the effect of APP treatment was greatlyreduced as the gases were blocked by the fabric surface; they bouncedoff and flew away from the fabric surface. At the same time, when thedistance reached 5 mm, the effect of APP treatment was greatly reduced

as the velocity and activity of active plasma species were greatlydecreased by the time they reached the fabric surface.

Although APP treatment with various processing parameters doescause yellowing effect on knitted wool fabrics (Table 1), based onboth visual and instrumental evaluation, yellowing effect on APP trea-ted knitted wool fabrics was not significant.

3.4. Shrinkage measurement

No fabric shrinkage was noted in APP treated knitted wool fabricbut about 5% area shrinkage was noted in the anti-pilling agent trea-ted knitted wool fabric. In the anti-pilling finishing process, wool fab-rics were immersed in water. Shrinkage clearly occurs because of therelaxation and felting shrinkages which are caused by the knitted fab-ric structure getting relaxed when immersed in water. Although verymild shaking was applied during the finishing process, felting be-tween wool fibre still occurred, leading to fabric shrinkage. In thecase of APP process, no water was used and hence no relaxation andfelting shrinkages occurred.

4. Conclusion

APP treatment can reduce pilling of knitted wool fabric significantlyand the experimental results reveal that wool fibre surface suffers vari-ous degrees of roughening, depending on parameters used. The etchingand rougheningeffect on thewoolfibre surface is themain reason for re-duced pilling. The results also show that APP treated knitted wool fabrichad the same anti-pilling grading as the conventional anti-pilling finish-ing agent. In addition, APP treated knitted wool fabric experienced noarea shrinkage but shrinkage was observed in conventional chemicallytreated samples. Meanwhile the anti-pilling property of knitted woolfabric was good and no significant yellowing was observed on the APPtreated knitted wool fabric surface.

Acknowledgements

Authors would like to thank the financial support from the HongKong Polytechnic University for this work.

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